1. Field of the Invention
The present invention relates to a method for producing a polyvinyl ester resin emulsion which has high viscosity and has good water resistant adhesiveness, heat-resistant adhesiveness, hot water-resistant adhesiveness and storage stability, especially high-temperature storage stability, and which has good film-forming ability and is readily formed into transparent films.
2. Description of the Related Art
Heretofore, polyvinyl alcohol (hereinafter referred to as PVA) is widely used as protective colloid in emulsion polymerization of an ethylenic unsaturated monomer, especially a vinyl ester monomer such as typically vinyl acetate. Aqueous emulsions of polyvinyl ester resins obtained through emulsion polymerization with PVA serving as protective colloid have many applications in various fields of adhesives for paper products, wood products, plastics, etc., binders for dip-coated paper, non-woven fabrics, etc., as well as admixtures, jointing agents, coating compositions, paper processing agents, fiber processing agents, etc.
Controlling the degree of hydrolysis of PVA therein makes it possible to change the physical properties of such aqueous emulsions, some of which will have low viscosity just like Newtonian fluid, and will be relatively highly resistant to water, while some others will have high viscosity relatively independently of ambient temperatures. Depending on such their properties, aqueous emulsions containing PVA have many applications in various fields.
For example, for adhesives for wood products, preferred are emulsions having higher viscosity, and widely used are aqueous emulsions of polyvinyl esters that contain partially-hydrolyzed PVA serving as protective colloid. Such aqueous emulsions of polyvinyl esters that contain partially-hydrolyzed PVA serving as protective colloid have good low-temperature storage stability and their viscosity can be increased with ease, but they have a problem in that their water resistance is poor. On the other hand, aqueous emulsions of polyvinyl esters that contain completely-hydrolyzed PVA serving as protective colloid have good water resistance, but have a problem in that their low-temperature storage stability is not good. To solve the problems, the inventors of Japanese Patent Laid-Open Nos. 46252/1988 and 62347/1989 have proposed a technique of adding 3-methyl-3-methoxybutanol or a water-soluble compound having an alcoholic OH group to such aqueous emulsions, and they have succeeded in improving the water resistantness and the storage stability of the resulting aqueous emulsions. However, since the aqueous emulsions proposed contain a water-soluble compound added thereto, their water resistance is limited, and, in addition, since their plasticizability that is indispensable to polyvinyl acetate emulsions is poor, they require some plasticizer at present.
The inventors of Japanese Patent Laid-Open Nos. 21529/1999, 21380/1999 and 226774/1998 have proposed a technique of using, as a dispersant, an ethylene units-having polyvinyl alcohol in emulsions such as those discussed herein, and they have succeeded in significantly improving the water resistance and the low-temperature storage stability of the emulsions that contain the dispersant. However, the solubility in water of the ethylene units-having polyvinyl alcohol is lowered by the ethylene units introduced into the polymer. Therefore, the emulsions that contain the polyvinyl alcohol serving as protective colloid have a problem in that they are greatly thickened while produced through emulsion polymerization at 60 to 80xc2x0 C. In case where emulsions are greatly thickened while they are produced through emulsion polymerization, they could not be well stirred and therefore could not be good emulsions. To solve the problem, the solid content of the emulsions must be lowered in order that the emulsions being produced could be well stirred even at high temperatures. As a result, at present, the emulsions thus produced shall inevitably have low viscosity, and therefore could not apply to adhesives for wood products that require high viscosity. In addition, the emulsions produced are also thickened at high temperatures of 40xc2x0 C. or higher, and their high-temperature storage stability is not good.
Emulsions that contain such an ethylene units-having polyvinyl alcohol serving as protective colloid have good low-temperature storage stability. Therefore, completely-hydrolyzed, ethylene units-having polyvinyl alcohol products are being used also in the field in which partially-hydrolyzed PVA has heretofore been used. However, the interfacial activity of the completely-hydrolyzed PVA is lower than that of the conventional partially-hydrolyzed PVA, and therefore the particle size of the emulsions that contain the completely-hydrolyzed PVA shall be large. As their particle size is large, the emulsions are problematic in that the films formed from them could not be transparent.
Given that situation, the present invention is to provide a polyvinyl ester resin emulsion which has high viscosity and has good water resistant adhesiveness, heat-resistant adhesiveness, hot water-resistant adhesiveness and storage stability, especially high-temperature storage stability, and which has good film-forming ability and is readily formed into transparent films.
We, the present inventors have assiduously studied to develop such a polyvinyl ester resin emulsion having the good properties as above, and, as a result, have found that a polyvinyl ester resin emulsion produced through emulsion polymerization of a vinyl ester monomer in the presence of a polyvinyl alcohol serving as protective colloid and in the presence of a water-insoluble, hydroxyl group-containing compound satisfies the above-mentioned object. In addition, we have further found that, when a polyvinyl alcohol containing from 0.5 mol % to 20 mol % of ethylene units is used in the method of emulsion polymerization, then the properties of the polyvinyl ester resin emulsion produced therein are more improved. On the basis of these findings, we have completed the present invention.